NIST to redefine the kilogram based on a fundamental universal constant (opens in new tab)

Four of the seven physical constants used to define the seven base SI units will change, so a bunch of physical constants that currently have exactly defined values will start being subject to measurement uncertainty. So μ_0 will no longer be exactly 4π × 10^-7 H/m, k_C = 1/4πε_0 will no longer be exactly 8,987,551,787.3681764 N m^2/C^2, and 1 mol of carbon-12 will no longer have a mass of exactly 12 g.

Slightly off-topic, but looking at the temperature section in your second link reminded me of the latest video from Linus Tech Tips[1] where he gets bashed for using the expression "degrees Kelvin". Personally, I don't see the problem. Since a kelvin is a hundredth of the difference between boiling and freezing temperatures of water, there is a notion of scale so the term "degrees" makes sense.

1. https://www.youtube.com/watch?v=60OkanvToFI

As Veritasium explains [0], all of the reference kilograms have drifted, and some appear to weigh less than they used to. So even if we know the precise mechanism of action of the drift, it doesn't help with the fact that our measurements are less reliable than they used to be.

Of course, what's probably happened is that our measuring tools have gotten more accurate!

[0] https://www.youtube.com/watch?v=ZMByI4s-D-Y

Do they account for potential variations in gravity where they've been doing the measurements? Given how ridiculously sensitive the equipment is the position of the moon probably has an impact on the readings they're getting.

> What's really need though is a universal, stable over eons, single standard for time, length, and mass. I believe time is N cycles of an excited sodium (light) emission. Length is N wavelengths of that same emission in a vacuum. Mass would be N atoms.

Turns out they know that :-)

The problem isn't definition, its the accuracy of reproducibility. You need a mechanism that can be built from scratch and can be believed to produce identical results. You can pull that off counting wavelengths of an unknown number of a known pure atom. Counting actual numbers of atoms is quite difficult. Typically we do it statistically...via mass! We could get it precisely by using a small number, but then actually measuring the mass (rather than calculating it) would be difficult.

A second (time) is already defined on cesium transition, a meter (length) is defined as a fraction of the distance light travels in a second. Both things which we can accurately measure for some time now, and which universal and stable over eons.

What is the problem that would be solved by switching to a single element?

They reference that silicon definition later on in the article.

It's kind of not important because nobody measures the kinds of things that require the level of precision applied to the kilogram definition using pounds.

Would be more important if the pound had an independent definition - suddenly, a lot of international trade and unit conversion apps are affected.

Indeed, all of the US units are defined in terms of metric standards. There is no standard inch.

Surely this is the point, though?

We rearrange the approximation: h = 6.626069934 x 10−34 kg∙m2/s to solve for kg and thus have our definition in terms of h?

Meters and seconds are already defined by physical constants, so once you know Planck's, you know kilograms.

Right - can somebody explain how this unit of kg-m2/s (weight diffusion? work-seconds?) can be used to define weight? Seem circular.

The constant is measured via experiments and because mass appears in the expression of the constant given above, a measure of the kg is obtained. I.e. the closer we approximate the constant (6.62...) the closer we get to determine what a kilogram actually weights (in relation to meters and seconds).

It seems you've found the source of the obesity crisis!

context?

Good question. Apparently, the unit (mass of 1 litre of water at the ice point) was supposed to be called "grave", but then it was considered that most measurements would be for much smaller amounts, and so it was switched to the gramme, but then for the definition they stuck to the original idea, now re-christened kg.

I don't quite understand this, as they could've defined it to be the mass of one cubic centimetre of water, rather than a cubic decimetre of water. Or they could've said that the unit is gram, and 1 g is 1/1000 of the mass of this artefact.

I've also read stories that revolutionaries objected to the name "grave", as it is close to Grave (French), Graf (German), that is, the title of nobility (anathema for the Republicans of the French Revolution). Thus, instead of 1 grave we have 1 kilogram.

At any rate, it was all rather messy and political, as the delightful book Whatever Happened to the Metric System?: How America Kept Its Feet by John Bemelmans Marciano recounts.

See also precisely this question at Physics StackExchange:

https://physics.stackexchange.com/questions/64562/why-metric...

Because it was easier to produce prototype of a bigger unit? And easier to measure it with lower error margin?

Because of how SI units evolved historically; basically, there was the "cgs" version of the metric system, and the "mks" version, and the latter won out. (Note that neither version has all three "base" units pure in terms of metric prefixes--the first has centimeters instead of meters, the second has, as you note, kilograms instead of grams.)

How to define fundamental units has always been a matter of precision measurements, not theoretical purity. If the standards body decides to base the definition on Planck's constant, instead of a certain amount of electrons, the rationale will be higher accuracy in measurements of Planck's constant than of the mass of electrons.

It's OK though, because the metre and second are already based on physical constants: the second is "the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom", and the metre is defined from the second by fixing the speed of light at 299 792 458 m/s.

FYI, from Wikipedia:

>For electrons or electron holes in a solid, the effective mass is usually stated in units of the rest mass of an electron, me (9.11×10−31 kg). In these units it is usually in the range 0.01 to 10, but can also be lower or higher—for example, reaching 1,000 in exotic heavy fermion materials, or anywhere from zero to infinity (depending on definition) in graphene. <

One way has been by counting silicon atoms in a nearly-perfect spherical crystal.

http://aip.scitation.org/doi/full/10.1063/1.4921240

The article explains, albeit it's maybe not very obvious if you aren't acquanited with metrology.

> So in 2014, at the quadrennial General Conference on Weights and Measures (yep, that's a thing), the scientific community resolved to redefine the kilogram based on Planck's constant, a value from quantum mechanics that describes the packets energy comes in. If physicists could get a good enough measure of Planck's constant, the committee would calculate a kilogram from that value.

> “But it's a very difficult constant to measure,” Pratt said. He would know: He and his colleagues at NIST have spent much of the past few years trying to come up with a number accurate and precise enough to please the finicky physics community.

Basically, it was hard to measure Planck's constant precisely enough to be as precise as the old standard. For compatibility reasons, the way this usually works is that they will measure Planck's constant and then define the kilogram so that `(k * Planck's constant) = (old mass of the kilogram)`, where `k` is whatever constant that makes this work out. To do this properly, you need to be able to measure Planck's constant with the same level of precision (and accuracy) as the old mass of the kilogram was known. Apparently this wasn't easy, presumably because Planck's constant is very small.

Defining your fundamental unit of mass in terms of a single arbitrary physical object costs a lot of theoretical purity, though it might not cause a lot of problems in practice. You mention a potential loss of precision. But we should gain accuracy by defining it in terms of fundamental constants. When the object itself changes, do we simply have a standard that drifts more than the physical constants of the universe? Do textbook publishers need to update all examples and problems using micrograms because the unit drift at that scale became significant? Should anyone without access to a reference object not get a real, accurate value for the kilogram? Do we keep using the mass value we know the reference object had, or does our kilogram really change with the object? Do we need to update any equations with a constant in them involving mass in any unit or derived unit in the equation once a year for high precision applications?

The problem with a physical standard like that is that you can't (easily) ship it to labs all over the world so that they can calibrate to it. At least when you use fundamental constants, each lab can set up their own equipment to produce the correct measurement.

Also, anything physical will shed atoms, which will affect the mass.

The simple platinum cylinder's margins of error are pretty much unbounded over time. If you come up with a good physical definition, our knowledge of the value will only get more precise over time as the means for measurement are improved.

It's the opposite: it makes sense practically, because it means a setup that can be reproduced "anywhere" based on pure knowledge, without depending on being able to ensure the continued integrity of a physical object that has been shown to change over time.

Most people can continue to depend on prototypes for calibration, so it does not cost us any complexity in that respect. What we're gaining is to be able to independently re-calibrate multiple prototypes.

I wouldn't say politically, but I'm sure that scientific aesthetics play a role. There are people who have devoted their careers to the development of fundamental standards. We search for the next digit of h, for the same reason why we search for the Higgs particle -- because we're curious.

A better kilogram might not have a practical use right away, but it might in the future, if for instance we want to do things like look for time dependent variations in the physical constants, tiny departures from existing theories of mechanics, etc.

> Ha, the irony! The USA's NIST defines a SI unit to the rest of the world; meanwhile, most of citizens don't know what it is.

...except actually it's the International Committee for Weights and Measures.

However I will help you retain your justifiable sense of ironic superiority: the US is one of the 17 original signatories to the metre convention (in May 1875: http://www.bipm.org/en/about-us/member-states/original_seven...). Also all the US conventional units have been based on the SI metre and kilogramme since 1959. And of course the metric system is familiar to any American in the military and/or who uses illegal drugs.

Although its not part of the BIPM, my favorite such standards organization is the International Earth Rotation Service (justified paying my taxes -- what if they stopped???). Sadly they recently renamed themselves "International Earth Rotation and Reference Systems Service"

Apropos of little: I used to live quite close (a couple of hundred metres) to an official metre, as there is one on the wall across the street from the French Senate. When the system was originally promulgated, markers were erected around France; you could bring something (piece of string or whatnot) and make your "own" metre to bring home and measure things. There are two or three of them still extant.

I would wager most US citizens are at least passingly familiar with the kilogram. Metric units are used in science courses, after all.

For those who missed it, the original submission title (from the article) was "Scientists are about to change what a kilogram is. That’s massive."

Just in case folks are unaware, this is the parapsychology guy that believes the following:

> Sheldrake's morphic resonance hypothesis posits that "memory is inherent in nature" and that "natural systems, such as termite colonies, or pigeons, or orchid plants, or insulin molecules, inherit a collective memory from all previous things of their kind" ... Sheldrake proposes that it is also responsible for "telepathy-type interconnections between organisms". His advocacy of the idea encompasses paranormal subjects such as precognition, telepathy and the psychic staring effect as well as unconventional explanations of standard subjects in biology such as development, inheritance, and memory.

The reason his TED talk was pulled is because he's a crank, and the entire talk is a confused defense of pure BS.